1. Field of the Invention
The present invention relates to methods of treating syndiotactic polypropylene, treated syndiotactic polypropylene, and to products made thereof. In another aspect, the present invention relates to methods of treating syndiotactic polypropylene to improve the crystallization rate and temperature, to syndiotactic polypropylene compositions having improved crystallization rate and temperature, and to products made thereof. In even another aspect, the present invention relates to methods of increasing the cell II (low melting form) structure content of syndiotactic polypropylene, to syndiotactic polypropylene compositions having increased content of cell II (low melting form) structure, and to products made thereof.
2. Description of the Related Art
Polypropylene has long been known to exist in several forms. Generally, isotactic propylene (iPP) can be described as having the methyl groups attached to the tertiary carbon atoms of successive monomeric units on the same side of a hypothetical plane through the polymer chain, whereas syndiotactic polypropylene (sPP) may generally be described as having the methyl groups attached on alternating sides of the polymer chain.
More specifically, the isotactic structure is typically described as having the methyl groups attached to the tertiary carbon atoms of successive monomeric units on the same side of a hypothetical plane through the main chain of the polymer, e.g., the methyl groups are all above or all below the plane. Using the Fischer projection formula, the stereochemical sequence of isotactic polypropylene is described as follows: ##STR1##
Another way of describing the structure is through the use of NMR spectroscopy. Bovey's NMR nomenclature for an isotactic pentad is . . . mmmm . . . with each "m" representing a "meso" dyad or successive methyl groups on the same side in the plane. As known in the art, any deviation or inversion in the structure of the chain lowers the degree of isotacticity and crystallinity of the polymer.
Syndiotactic polymers, in contrast to the isotactic structure, are those in which the methyl groups attached to the tertiary carbon atoms of successive monomeric units in the chain lie on alternate sides of the plane of the polymer. Using the Fischer projection formula, the structure of a syndiotactic polymer is designated as: ##STR2##
In NMR nomenclature, this pentad is described as . . . rrrr . . . in which each "r" represents a "racemic" dyad, i.e., successive methyl group on alternate sides of the plane. The percentage of r dyads in the chain determines the degree of syndiotacticity of the polymer. Syndiotactic polymers are crystalline and, like the isotactic polymers, are insoluble in xylene. This crystallinity distinguishes both syndiotactic and isotactic polymers from an atactic polymer which is soluble in xylene. Atactic polymer exhibits no regular order of repeating unit configurations in the polymer chain and forms essentially a waxy product.
Clarity, toughness and elasticity are some of the extraordinary advantages offered by syndiotactic polypropylene over some other semi-crystalline polyolefins such as isotactic polypropylene.
However, the full potential of syndiotactic polypropylene, either in the unstabilized form or the stabilized form against degradation, cannot be realized due to problems associated with low crystallization rates and low crystallization temperatures.
While the heat of crystallization of a commercially produced isotactic polypropylene is about 100 Joules per gram, the heat of crystallization of syndiotactic polypropylene with greater than about 70 per cent racemic pentad, is only about 30 to about 50 Joules per gram. Thus, sPP retains less crystallinity than iPP. The crystallization rate of sPP is significantly slower than iPP. The sPP continues to crystallize even after pelletization during the continuous operation. Low crystallization temperatures (T.sub.c) also require cooling of injection molded parts or extruded films or sheets to much lower temperatures than needed, for example, for isotactic polypropylene. This results in slower production rates and increased energy costs.
In addition, syndiotactic polypropylene has been shown to exhibit polymorphism, as described, for example in A. J. Lovinger et al., "Morphology and Thermal Properties of Fully Snydiotactic Polypropylene", Macromolecules, 27, 6603-6611 (1994). X-ray diffraction data, electron diffraction data, and DSC ("Differential Scanning Calorimeter") curves, all have shown that the crystal structure of syndiotactic polypropylene may contain right handed helices along the "a" and "b" crystallographic axes, in which case the type of structure is denoted "cell I" type. A "cell II" (or low melting form) type structure contains anti-chiral helices along "a" crystallographic axis and chiral helices along "b" crystallographic axes. "Cell III" (or high melting form) type of syndiotactic polypropylene structure contains anti-chiral helices along both "a" and "b" crystallographic axes. Correlation of DSC melting peaks with the x-ray and electron diffraction results show that cell II (low melting form) type structure corresponds to the lower melting peak in DSC, whereas cell III (high melting form) type structure corresponds to the higher melting peak of syndiotactic polypropylene. In almost all cases, a non-nucleated and non-annealed sample of syndiotactic polypropylene shows the presence of two melting peaks by DSC. Without being limited by theory, it is believed that cell II (low melting form) type structure is kinetically controlled and is formed at rapid rates, whereas cell III (high melting form) type structure is the thermodynamically more stable structure and is formed more slowly. Cell III (high melting form) type structures have also been shown by Lovinger, in the aforementioned reference, to be prone to development of microcracks.
Finally, not only does syndiotactic polypropylene have crystallization temperatures and rates that could be improved, some common processing additives, such as calcium stearate, tend to reduce the crystallization rate even more.
Thus, in spite of the advancements in the prior art relating to syndiotactic polypropylene, there is a need for a method of improving the crystallization rate and temperature of syndiotactic polypropylene.
There is another need in the art for a syndiotactic polypropylene having improved crystallization rate and temperature of syndiotactic polypropylene.
There is even another need in the art for products made from syndiotactic polypropylene having improved crystallization rate and temperature of syndiotactic polypropylene.
There is still another need in the art for methods of increasing the cell II (low melting form) content and decreasing the cell III (high melting form) content of syndiotactic polypropylene, and for such polymers and products made therefrom.
These and other needs in the art will become apparent to those of skill in the art upon review of this specification, including its drawings and claims.